In telecommunication systems, for example, data processors intervening in the establishment and eventual termination of a connection are frequently duplicated in order to prevent interruption of service if one processor should fail. The two processors, in normal operation, simultaneously perform a succession of operating cycles which may be considered steps of a macroprogram involving a series of instructions, each instruction relating to the reading of a data word at a specified address in the data store or the writing of a new data word in place of an old one at such address.
For proper performance it is necessary to check from time to time, preferably during every cycle, the operations of the two data processors in order to ascertain whether they are progressing identically or not. Prior checking systems of this character utilize special-purpose equipment which, because of its limited field of use, is expensive as to both the hardware and the software employed.